1. Field of the Invention
The present invention relates to a technique for detecting optical characteristics of an image forming apparatus using an electrostatic or electrophotographic recording method.
2. Description of the Related Art
In general, an electrophotographic image forming apparatus includes an optical scanning apparatus that drives a semiconductor laser in accordance with input image data to form an electrostatic latent image corresponding to the image data on a photosensitive member.
The semiconductor laser used as a light source of the optical scanning apparatus emits a laser beam having a wavelength-temperature characteristic. In other words, when the temperature varies, the wavelength of the laser beam also varies. Accordingly, refractive and reflective indices of lenses for passing the laser beam therethrough and mirrors for reflecting the laser beam in the optical scanning apparatus vary. As a result, the magnification of scanning lines formed on the photosensitive member by the laser beam also vary.
When a multibeam light source is used, the above-described variation in magnification caused by the temperature characteristic is added to the initial differences in magnification due to differences in wavelength between the laser beams. As a result, magnifications of all of the laser beams vary individually. Accordingly, a method for detecting the magnification of each laser beam by forming test patterns on a photosensitive member and measuring gaps between the test patterns on an intermediate transferring member has been suggested (refer to, for example, Japanese Patent Laid-Open No. 8-156332).
However, the number of light sources may be increased to several tens or hundreds to achieve image forming apparatuses with higher speeds and resolutions. In such a case, it takes an extremely long time to form the test patterns and detect them. Therefore, the above-mentioned method is considered impractical.
The magnification of each beam is desired to be detected in the optical scanning apparatus instead of on the intermediate transferring member. Accordingly, a method for detecting the magnification by arranging sensors at upstream and downstream positions of the scanning lines and measuring the time required to scan between the two positions has been suggested (refer to, for example, Japanese Patent Laid-Open No. 2002-122799).
However, in the case in which, for example, a slope of the scanning lines caused by errors generated when the optical scanning apparatus is attached to the image forming apparatus is corrected in the optical scanning apparatus, accurate magnifications cannot be detected simply by measuring the time required to scan between the two position. The reasons for this will be explained below with reference to FIGS. 5, 6A-B, and 7A-B.
FIG. 5 is a schematic top view of the main part of an optical scanning apparatus. FIG. 5 shows scanning-position detection sensors 91 and 92, laser beams A and B emitted from a multibeam light source, a polygonal mirror 33, and an f-θ lens 34. FIG. 6A illustrates the relationship between the scanning-position detection sensors 91 and 92 and the laser beams A and B of the multibeam light source shown in FIG. 5 (case in which a slope θ is zero (0) in FIG. 7A).
FIG. 6B illustrates signals detected by the scanning-position detection sensors 91 and 92. FIG. 7A illustrates the relationship between the scanning-position detection sensors 91 and 92 and the laser beams A and B having a slope with an angle θ. FIG. 7B illustrates signals detected by the scanning-position detection sensors 91 and 92 when the scanning lines have a slope with the angle θ.
When the laser beams A and B have different wavelengths, the mirror 33 and the f-θ lens 34 have different reflective and refractive indices, respectively, for the laser beams A and B. Therefore, as shown in FIG. 5, the laser beams A and B have different scanning line widths (scanning magnifications). In the case shown in FIG. 5, the scanning speed of the laser beam B is higher than the scanning speed of the laser beam A. Therefore, in FIG. 6B, times Ta and Tb required for the laser beams A and B, respectively, to move between the scanning-position detection sensors 91 and 92 satisfy Ta>Tb.
When the distance between the scanning-position detection sensors 91 and 92 is L and the scanning speeds of the laser beams A and B are Va and Vb, respectively, the following equations are satisfied:Va=L/Ta, Vb=L/Tb 
When the scanning magnification of a scanning line A′ of the laser beam A is defined as 1, the scanning magnification of the scanning line B′ of the laser beam B is calculated as follows:Vb/Va=Ta/Tb 
However, if a slope of the scanning lines caused by errors generated when the optical scanning apparatus is attached to the image forming apparatus is corrected by adjusting lens positions and the like in the optical scanning apparatus, the scanning lines have a slope with an angle θ relative to the scanning-position detection sensors 91 and 92, as shown in FIG. 7A.
In FIG. 7A, the length of the scan lines is determined as L, although it is L′=L/cosθ in practice. The actual scanning speed of the laser beam A is calculated as follows:Va′=L′/Ta=L/(Ta cos θ)
However, the scanning speed of the laser beam A is determined as follows:Va′=L/Ta 
This means that the determined scanning speed has an error by a factor of (1/cos θ−1). When the scanning speed of the laser beam A is used as a reference, the scanning magnification of the scanning line B′ is determined using the following equation:Vb′/Va′=Ta/Tb 
Therefore, although relative magnifications can be calculated, magnifications of all of the laser beams include errors because the magnification of the scanning line A′ used as a reference includes an error.